Lithium ion secondary batteries typical of non-aqueous electrolyte secondary batteries have high electromotive force and high energy density. Thus, lithium ion secondary batteries are in an increasing demand as a main power supply of mobile communication devices and portable electronic devices.
For the development of lithium ion secondary battery, it is an important technical issue to enhance the reliability of the battery. A positive electrode of lithium ion secondary battery contains a lithium composite oxide such as LixCoO2 or LixNiO2 (x is changeable depending on charge and discharge of battery). These lithium composite oxides contain Co4+ or Ni4+ having high valence and reactivity at the time of charging. Due to this, decomposition reaction of electrolyte pertaining to lithium composite oxide is accelerated in a high temperature environment, generating a gas in a battery. Consequently, sufficient cycle characteristic or storage characteristic at high temperature cannot be obtained.
In order to inhibit the reaction between an active material and an electrolyte, it is suggested to treat the surface of a positive electrode active material with a coupling agent (Japanese Patent Laid-Open Nos. 11-354104, 2002-367610, and 8-111243). This allows a stable coating to be formed on the surface of active material particle. Therefore, the decomposition reaction of electrolyte pertaining to lithium composite oxide is inhibited.
Further, from the viewpoint of inhibiting the reaction between the active material and the electrolyte and improving cycle characteristic and storage characteristic at high temperature, addition of various elements to a positive electrode active material is suggested (Japanese Patent Laid-Open Nos. 11-16566, 2001-196063, 7-176302, 11-40154, and 2004-111076).
Furthermore, improvement of water resistance is a problem regarding LixNiO2. Therefore, it is suggested that a coupling agent is used to make the surface of LixNiO2 hydrophobic so that the stability of the active material is enhanced (Japanese Patent Laid-Open No. 2000-281354).
The technology for inhibiting gas generation by using a coupling agent has the following points to be improved. Many of lithium ion secondary batteries are used for various portable devices. Various portable devices are not always used immediately after battery charge is completed. There are many cases wherein a battery is kept in charge condition for a long period, and thereafter discharge starts. However, the cycle life characteristic of battery is generally evaluated under a condition different from such practical use condition.
For example, a standard cycle life test is conducted in a condition wherein a short rest (intermission) period is given after charging. The rest period is about 30 minutes, for example. If such condition is applied for evaluation, the cycle life characteristic can be improved to some extent by conventionally suggested technologies.
However, it is necessary to consider that intermittent cycles are repeated on the assumption of practical use conditions. If charge-discharge cycles are repeated with a longer rest period (e.g. rest period of 720 minutes) after charging, the above technologies cannot provide sufficient life characteristic. In other words, a conventional lithium ion secondary battery still has a problem, that is improvement of intermittent cycle characteristic.